Allelopathic interference of invasive Acacia dealbata Link on the physiological parameters of native understory species

The tree Acacia dealbata Link is an Australian woody legume that has become a serious environmental problem in Northwest Spain, where its expansion is assumed to reduce populations of native species and threaten local plant biodiversity. In order to investigate the potential involvement of allelopathic mechanisms in this process, net photosynthetic and respiration rates of four test native understory species (Hedera hibernica (G. Kirchn.) Bean, Dicranum sp., Dactylis glomerata L. and Leucobryum sp.) were evaluated using a Clark-type electrode in the presence of canopy leachate collected under A. dealbata stands at four times of the year for 2 years and macerate from their apical branches at the same sites and times. The first two test species were present both inside and outside of A. dealbata stands, while the last two were only located outside the stands. We found that there were significant differences in respiration and net photosynthetic rates between the control and A. dealbata extracts in all test species. The respiration rate was increased by both canopy leachate and macerate extracts on certain collection dates, but net photosynthetic rate was stimulated by macerate and inhibited by canopy leachate on other dates. The main phenological stages of A. dealbata in which respiration and net photosynthetic rates were more affected were blossoming, inflorescence formation and in periods after severe drought, in this decreasing order. Our results also showed that Dicranum sp., Leucobryum sp. and D. glomerata were more affected by aqueous extracts than other species during a 2-year period. We suggest that the observed inhibitory or stimulatory effects on the physiological parameters studied could have an adverse effect on the understory species, and that allelopathic interference seems to participate in this process.